Mayonnaise and dressings

Why

Mayonnaise is an emulsion: millions of tiny oil droplets dispersed in the little water that comes from the yolk and the lemon juice or vinegar. To stop the droplets merging again, they get coated with emulsifiers from the yolk – lecithin and other phospholipids together with proteins; mustard helps a little too. At the start there is so little water that it can't break a big splash of oil into droplets or coat them, so the oil goes in drop by drop at first and faster only once the emulsion has thickened. Cold ingredients come together a little less readily, so it helps if the yolk and oil are at the same temperature, ideally room temperature – and a mayonnaise that is very stiff with a greasy sheen is short of water, not of yolk. An oil-and-vinegar dressing is only a temporary emulsion: mustard or honey hold it together for a while, but sooner or later it separates.

How to do it

• Take the yolk, oil and mustard out of the fridge about half an hour ahead – at room temperature the emulsion forms more readily. Don't leave raw yolk standing any longer.
• Whisk the yolk with a teaspoon of mustard and a little vinegar or lemon juice before any oil goes in.
• Add the oil literally drop by drop at first, whisking constantly; speed up only once it thickens.
• If the mayonnaise is stiff and looks greasy, whisk in a teaspoon of water or lemon juice.
• Raw yolk can carry salmonella even from a fresh egg: pasteurised yolk is safest, and never serve raw-yolk mayonnaise to pregnant women, young children, older people or the immunocompromised.
• Put the finished mayonnaise straight in the fridge and eat it within 24 hours; shake or whisk a dressing again just before serving.

If it goes wrong

Broken mayonnaise can be saved: put a fresh yolk, or a spoonful of water or mustard, into a clean bowl and whisk in the broken mixture drop by drop, as if it were oil. Once the emulsion takes, add the rest faster. Never the other way round – stirring a new yolk into the broken sauce won't help.